1. Field of the Invention
This invention is in the field of corrosion and abrasion resistant coatings formed on metal, glass, or composite substrates, for example, aluminum alloys.
2. Background
The prevention, or reduction, of corrosion is of concern with most types of metals, including aluminum and steel which find broad use in, for example, the automotive and aviation industries. Corrosion will significantly affect the appearance and integrity of such metals and, if left unchecked, may even destroy a piece of metal. Accordingly, there are continual efforts in the field to improve on prior art anti-corrosion techniques.
One technique for reducing the occurrence of corrosion involves passivating the surface of a metal with a heavy chromate treatment. While such treatments have proven effective, unfortunately chromium is highly toxic, believed to be a carcinogen, and environmentally unfriendly. Recently, OSHA has determined that chromate-containing aerosols, such as those generated by large scale solution spraying constitute a serious health threat for workers that are exposed to such operations. Thus, the use of chromate is altogether undesirable.
Several anti-corrosion schemes have been suggested which avoid the use of chromate entirely. Of these, coatings comprising silane have shown a great deal of promise. However, such coatings are still evolving and suggested schemes typically attempt to balance a number of limitations such as adhesion to the substrate material, compatibility with paints, scratch resistance, abrasion resistance, immunity to solvents, and others.
Organically modified silicates (Ormosils), are well known hybrid organic-inorganic materials. Typically these materials are formed through the hydrolysis and condensation of organically modified silanes, Rxe2x80x94Si(OX)3, with traditional alkoxide precursors, Y(OX)4, where X may represent, for example, CH3, C2H5, C3H7, C4H9 and Y may represent, for example, Si, Ti, Zr, or Al. R may be any organic fragment such as methyl, ethyl, propyl, butyl, isopropyl, methacrylate, acrylate, vinyl, epoxide, and the like. As used herein, the term xe2x80x9cOrmosilxe2x80x9d encompasses the foregoing materials as well as other organically modified ceramics, sometimes referred to as Ormocers. Ormosils are often used as coatings where an Ormosil film is cast over a substrate material through, for example, the sol-gel process.
Ormosil films are often found in high-performance applications which require abrasion resistant, anti-soiling, and anti-fogging coatings. In addition, when used over metal substrates, Ormosil films have been found to form a dense barrier to the penetration of corrosion initiators and thus provide good corrosion resistance. Ormosil films are of particular interest in the area of corrosion resistance because they exhibit mechanical and chemical characteristics of both their organic and inorganic networks to produce films which are durable, scratch resistant, adherent to metal substrates, flexible, dense, and functionally compatible with organic polymer paint systems. As a result, Ormosil coatings have been found useful for the protection of metal surfaces from a variety of aggressive environments.
It should be noted that the sol-gel process is likewise known in the art and consists essentially of hydrolysis and condensation reactions originating from alkoxide and/or silane precursors to form a polymeric network. Simplified chemical reaction sequences are given by:
(1) Hydrolysis:
Rxe2x80x94Si(OX)3+Si(OX)4xe2x86x92Rxe2x80x94Si(OH)3+Si(OH)4+7XOH 
(2) Condensation:
Rxe2x80x94Si(OH)3+Si(OH)4xe2x86x92Rxe2x80x94Si(OH)2xe2x80x94Oxe2x80x94Si(OH)3+H2O 
where: R=vinyl, methacrylate, epoxide, etc.
X=alkyl functionality or fragment.
Due to the rapid hydrolysis and condensation rates of metal or non-metal alkoxides derived from elements other than silicon in Ormosil preparation methods, the alkoxides typically form porous sub-micron sized particles within the ormosil matrix. These coatings are thin, generally less than 2-4 microns. While coatings prepared in this fashion provide good abrasion resistance characteristics, the air-sensitive nature of the parent alkoxides makes this coating preparation method less than desirable. A need thus exists for an Ormosil coating prepared from alkoxides which are not air sensitive and which incorporates inorganic particles of a larger size in order to produce a thicker, mechanically stable, film.
It is thus an object of the present invention to provide Ormosil films for enhanced abrasion and corrosion resistance of metal substrates.
The invention described herein provides composite Ormosil coatings having inorganic particles generally of a size ranging from about 1 micron to about 75 microns entrapped therein. Unlike prior art coatings, dense particles are added to the Ormosil solution, rather than formed in-situ during the sol-gel reactions. The size of the particles is substantially larger than that of the particles formed in the preparation of prior art Ormosil coatings and allows the preparation of a mechanically stable thicker film. Examples of suitable inorganic particles include oxides, nitrides, carbides, carbonitrides, and the like. The inventive coating thus improves the corrosion and abrasion protection over that of the underlying substrate material. In a preferred embodiment, the Ormosil film is applied to the substrate material through a sol-gel process wherein the application method may include brushing, sponging, spinning, dipping, spraying, or other liquid application method.
In another embodiment of the present invention there is provided a process for improving the corrosion resistance and abrasion resistance of metals prone to corrosion by application of an organic-inorganic coating system. The coating system comprises an Ormosil composite coating having inorganic particles entrapped therein. Corrosion resistance is obtained based on barrier properties of the Ormosil film. The films are prepared by mixing various silanes and organically modified silane reagents with acidified water. After stirring, the inorganic particles are added and the Ormosil material is deposited onto the surface of a substrate material. Thus, the present invention encompasses corrosion/abrasion protection based on the use of Ormosil films which incorporate inorganic particles as a coating for metal substrates. Such coating optionally may be used in conjunction with a conversion coating process.
In yet another embodiment of the present invention there is provided an abrasion resistant coating for a broad range of materials such as metals, glass, plastic, polymers, etc.
A better understanding of the present invention, its several aspects, and its advantages will become apparent to those skilled in the art from the following detailed description, taken in conjunction with the attached drawings, wherein there is shown and described the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated for carrying out the invention.